1. Field
This invention relates to the production of energetic materials. More specifically, the invention relates to means and methods for size reduction of particulate explosive materials.
2. State of the Art
Unplanned ignition of munitions is a hazard wherever munitions are made, stored, or used. Such ignition may occur due to fire, bullet or fragment impact, a shaped charge jet, or sympathetic detonation. To reduce this hazard, various means are being sought to decrease the sensitivity of energetic materials to such stimuli. Such materials are termed insensitive munitions (IM).
It has been discovered that explosive materials including RDX (cyclotrimethylene-trinitramine) and HMX (cyclotetramethylene tetra-nitramine) exhibit a decrease in sensitivity when the particle size from which the charge is formed is reduced below some threshold value, e.g. 2-7 microns. It is believed that other newly developed energetic materials having an inherent low sensitivity may also be made further insensitive by use of very small particle size. Such materials include NTO (3 nitro-1, 2, 4-triazol-5-one) and ADNBF (7-amino-4, 6-dinitrobenzofurozan).
Class 1 RDX and Class 5 RDX are specified according to military specification MIL-R-398C as having the following particle size distributions:
______________________________________ Through U.S. Standard Percent Passing Through Sieve Sieve No. Class 1 RDX Class 5 RDX ______________________________________ 20 (840 microns) 98 .+-. 2 -- 50 (297 microns) 90 .+-. 10 -- 100 (149 microns) 60 .+-. 30 -- 200 (74 microns) 25 .+-. 20 -- 325 (44 microns) -- 97+ ______________________________________ The approximate mean particle sizes, based on volume, for each class are: Class 1: 100-200 microns Class 5: 3-10 microns
Class 1 RDX is prepared as a wet slurry. It is shipped and stored as a 75 percent solids slurry in a water-alcohol mixture, e.g. 40 percent isopropyl alcohol and 60 percent water.
Class 5 RDX is generally prepared from crude RDX by recrystallization and is more resistant to unintentional ignition because of the reduced particle size.
The most common method of preparing finely ground RDX and HMX is to thoroughly dry the wet crystallized material, typically to less than 0.1 percent moisture and perform size reduction with a fluid energy mill. Inadequate grinding of RDX and HMX is generally believed to occur when the material contains even small quantities of moisture.
Size reduction of other wet materials in a fluid energy mill is not considered feasible unless the elastic carrier fluid is at a temperature where the water-containing liquid is evaporated from the particles. For an energetic slurry of 10-25 percent solids, the required temperatures of the carrier fluid for wholly evaporating the water-alcohol mixture are unsafe and may result in detonation. Thus, the energetic material is predried at a lower temperature. Furthermore, dry grinding of explosive materials carries with it inherent risks of detonation. In addition, the drying process is very energy intensive and thus, costly to operate. Detonation of nitramines has been known to occur in the drying step.
The other common method for producing RDX, HMX, or CPX of smaller particle size is a recrystallization process. The original crystallization of these materials necessarily results in large particle size. The energetic material such as Class 1 RDX, for example, is produced as a highly wetted material at about 20 percent solids. A solids concentration of 80-85 percent in a water-alcohol solution is used for safe shipment and storage.
Recrystallization is typically conducted by dissolving the energetic material in a solvent such as cyclohexanone or acetone and precipitation by quenching with water. The recrystallization process is both tedious and excessively consumptive of both time and energy. The use of toxic volatile solvents presents well-known environmental hazards. One disadvantage of the recrystallization process is the wide range of resulting particle sizes.
A process is needed for rapidly, safely, and inexpensively producing energetic materials of fine particle size, i.e. less than about 5-10 microns. In addition, a process is needed for producing a finely ground energetic material which has a more uniform particle size.